Because a light emitting diode (LED) has the advantages of long lifespan and high energy utilization rate, it has become a new light source to replace conventional light sources. The color of the light emitted by the LED corresponds to the frequency of the light, which may correspond to the band gap of the semiconductor material. InGaN is a suitable material for a light emitting layer in an LED emitting blue light, green light or purple light. The band gap of GaN is about 3.4 eV, and may be modulated from about 0.7 eV to about 3.4 eV by doping indium.
An epitaxial wafer for an LED may comprise a buffer layer, an N-type semiconductor layer, a multi-quantum well layer and a P-type semiconductor layer which are grown successively. The N-type and P-type semiconductor layers may comprise III-V group compounds, such as GaN or GaAs. A GaN LED is described for illustration purpose in this specification.
Conventionally, an epitaxial wafer for an LED may be manufactured in a metal organic chemical vapor deposition (MOCVD) device. The method for manufacturing the epitaxial wafer may comprise: growing a buffer layer on a top surface of a sapphire substrate through epitaxial technology; growing an N-type GaN layer on the buffer layer through epitaxial technology; growing a multi-quantum well layer on the N-type GaN layer; and growing a P-type semiconductor layer on the multi-quantum well layer.
The wavelength of an LED light may be determined by the content of indium in the multi-quantum well layer. For a multi-quantum well layer containing about 13% indium by weight, the growth temperature may be about 750° C. The indium content may be very sensitive to the growth temperature. When the growth temperature is changed by about 1 K, the indium content may be changed by about 0.3%, and the emission wavelength of the LED may be changed by about 1.5 nm. The MOCVD device may be bottom heated, so that a temperature gradient may be formed from the bottom to the top in the cavity of the MOCVD device. The temperature may thus decrease from the lower part to the upper part of the cavity, and a 2 K temperature difference may exist between the top surface and the lower surface of the sapphire substrate. Due to the temperature difference, the lower surface of the substrate with a higher temperature may undergo a larger thermal expansion, and consequently the substrate may warp upwards and be deformed to a bowl shape from a plate shape. Due to the presence of the temperature gradient, the warped epitaxial wafer may have a height difference of about 10 μm and a temperature difference of about 7 K between the center and edges thereof. Therefore, in the bottom heated MOCVD device, the wavelength of the light emitted from the center and the edge of the epitaxial layer may have a difference of 1.5×7=10.5 nm. In other words, the LED chip comprising the epitaxial wafer may have a poor light uniformity. Therefore, during the manufacture of LED chips, after grinding, polishing, dicing and chipping, point measurement and separation may be needed to obtain LED chips with different wavelengths, which may increase the complexity and cost of the manufacturing processes.